The present invention generally relates to a system for providing cooling flow to the turbine components of a gas turbine and, more particularly, to a system for providing a variable turbine cooling flow to the turbine at various power and altitude conditions.
A conventional turbine rotor cooling system may be designed to satisfy cooling needs of the high pressure turbine (HPT) blade and of the turbine disk at maximum turbine inlet temperature and compressor discharge conditions. This condition usually occurs at maximum engine power. At lower power settings, the turbine coolant air and gaspath temperatures are much lower. Coolant air temperature is lower at altitude than at sea level, even when gaspath temperature is high. In these cases, since the cooling circuit geometry is fixed the coolant flow rate requirements are less than the actual flow rate. The source of the cooling air is compressor discharge air. Use of an excess of this air, because it has had compression work done on it by the engine, adversely impacts the engine performance. For many applications, the desire to minimize fuel burn is most critical at lower power levels, because more operating time is spent at lower power levels as compared to time spent at high power.
Referring to FIG. 1, there is shown a conventional HPT cooling system. Turbine coolant air, 100, is delivered from the compressor section of the engine. Compressor discharge air flows as indicated by arrow 102 passes through passages 104. After passing through passages 104, the cooling air is divided into three separate cooling flows, as indicated by arrows 105, 106, 108. Cooling flow 105 provides cooling air for a downstream turbine disk (not shown). Cooling flow 106 provides cooling air for the high pressure turbine disk 110 and turbine blade, 112. Cooling flow 108 provides cooling air for the sealplate forward of the turbine disk.
U.S. Pat. No. 5,996,331 discloses a passive system in which a modulating device is arranged in the engine such that pressures generated within the engine may naturally activate a bellows which axially positions a sleeve over a flow control restriction. This passive system relies on pressures generated in the engine to naturally activate the bellows. There is no active control relationship between the temperature and/or power level angle and the amount of cooling flow through the turbine.
U.S. Pat. No. 4,217,755 discloses a control valve for effecting flow modulation using plenum discharge/ambient pressures for flow control. Axial annular member 140 acts as an axial spring to push leaf spring 144 axially closed or pull leaf spring axially open (FIG. 1). Annular member 140 and leaf spring 144 are assembled in a configuration that is prone to vibration oscillation and potential “Hemholtz” cavity resonation which may cause vibrational fatigue in gas turbine engines. The radial cooling over the leaf spring end 152 may also result in “reeding” and increased noise in the engine. In addition, this system has very little vibrational damping. This could result in component fatigue and cooling flow oscillation as the leaf spring system resonates. This patent requires that flow migrate past the flow control leaf spring outer edge 152 to provide full turbine cooling flow. Additionally, this patent uses a large radial leaf spring 144 to effect flow modulation. Many applications do not have the radial space to incorporate this size of an assembly. Further, this patent requires evacuation of the undercut cavity 130 to produce the required pressure differential across the leaf spring to open the flow circuit and adequately cool the turbine at high temperature conditions. Thus, if the exhaust circuit 164 gets plugged with dirt, compressor abraidable, etc. then the system may fail to deliver acceptable turbine cooling.
As can be seen, there is a need for an improved turbine cooling flow system that varies the amount of coolant flow being delivered to the turbine blades and disk at low power conditions.